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other hand, are not as well understood. If silver is transported as a sulfide complex,

boiling should also cause silver to precipitate. However, if silver is transported as a

chloride complex boiling is less likely to result in silver precipitation. Recent

experimental studies by Pokrovski et al. (2008) at 350 – 500°C (temperatures somewhat

higher than those in the epithermal environment) showed that the solubility of silver in

chloride solutions decreases with increasing pH, reflecting a change from transport as a

chloride complex in acidic solutions to transport as S-complexes in neutral to basic

solutions. Thus, the changes in solution properties described above as a result of boiling,

specifically an increase in pH, will also promote silver deposition if silver is transported

as a sulfur complex.

In this study, the relationship between fluid inclusion and mineral textural

evidence for boiling and precious metal grades in the classic epithermal Ag-Au deposits

at Guanajuato, Mexico, have been investigated to test for systematic correlations that may

be used in exploration for similar deposits. Approximately 850 samples were collected

from surface outcrops, underground workings, and recent drill core over a strike length of

4 km and to depths of 750 m beneath the surface in four active mines and one closed

mine along the Veta Madre. Each sample was assayed for Au, Ag, Cu, Pb, Zn, As, Sb.

The goal of this study was to develop a method to evaluate the economic potential

samples based only on data obtained during petrographic examination of thin sections

and does not require time-consuming and expensive microthermometry or microanalysis

of the inclusions.


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